The prior art shows years of concern over the production of vibration resistant locking fastener combinations. Such expedients as lock washers, cotter pins, glue bonds, telescopically wedging surfaces and mechanical deformation have been used to prevent or resist loosening of the elements of a threaded fastener combination.
I have discovered that the key to a vibration resistant self-locking fastener combination lies in the realization of firm radial contact between the male and the female thread forms such that all lateral movement is constrained under vibration conditions. By eliminating the possibility for lateral shifting or relative movement between the thread forms the high friction condition which is characteristic of axial tension is maintained and the tendency for the threaded elements to unwind is minimized. I have embodied this concept in the device disclosed in my U.S. Pat. No. 4,150,702 which issued on Apr. 24, 1979. In my patent I disclosed a prevailing torque nut for use in combination with a standard bolt or stud and in which vibration resistance is achieved through firm radial engagement between the crest of the bolt thread and the root of at least a portion of the nut. Specifically, the nut thread exhibits normal thread depth over a portion of its length so as to easily accommodate the bolt thread therein, but a substantially reduced depth over the remainder of its length such that threading the elements fully together progressively drives the bolt thread crest against the truncated flat of the nut. As this occurs material is worked from the top of the bolt crest and the included angle of the nut flanks is made smaller than the included angle of the bolt flanks so as to provide clearance for material which is removed from the bolt thread form crest during the threading process.
Because the locking effect in the combination described above often involves non-elastic deformation of the bolt thread, repeated use of the nut through removal and reapplication is likely to cause a significant loss of vibration resistance and prevailing torque level. Accordingly, the prevailing torque nut fastener of the aforementioned patent is recommended only in those applications where the nut can be substantially permanently applied.